This invention relates generally to blood chemistry monitoring systems and, more particularly, to a method and apparatus for reducing the volume of fluid required to purge a patient's blood from a blood sampling site after the blood has been sampled for measurement.
The invention is particularly suitable for use with a system that infuses a fluid into a patient substantially continuously, wherein some provision is made in the system for blood chemistry measurement. Such infusion delivery systems are connected to a patient at an intravenous (IV) port, in which a hollow needle/catheter combination is inserted into a blood vessel of the patient and an infusion fluid is introduced into the vessel at a controlled rate from an IV bottle or other source of fluid via a fluid flow line or infusion tube. The rate of infusion may be controlled by a simple manual clamp or by an electromechanical infusion pump or controller acting on the fluid flow line.
It is common practice to remove blood from a blood vessel for analysis while the patient is undergoing IV infusion. Typically, a mixture of blood and infusion fluid is removed from a blood sampling port in the fluid flow line with a first syringe to allow undiluted blood to reach the stopcock (?). A second syringe is used to take a blood sample to the laboratory for analysis. The fluid flow line is then purged with the infusion fluid to flush the blood in the fluid flow line back into the patient. The amount of infusion fluid required to flush the fluid flow line clean is a function of purge rate and the amount of blood in the fluid flow line. In a typical infusion delivery system of this type, it is not unusual to flush the line at a rate greater than 1,000 ml/hour with more than 6 to 10 times of the volume of he blood to be flushed back into the patient.
In recent years, automatic blood chemistry monitoring systems have been developed which may be combined with infusion delivery systems of this kind, using the IV port to periodically withdraw a blood sample, perform measurements of various characteristics of the blood, and then reinfuse the blood into the patient. The system then resumes normal delivery of the infusion fluid. Such a system is described in U.S. Pat. No. 4,573,968. In that system, an infusion pump normally pumps a suitable infusion fluid via an infusion tube and catheter into a patient, but intermittently reverses its direction, to draw a sample of blood from the patient through the catheter and into a sensor assembly connected to the infusion tube. The physical configuration of one suitable sensor assembly is disclosed in U.S. Pat. No. 5,165,406. The sensor assembly includes a plurality of sensors that produce electrical signals corresponding to various conditions or parameters of the patient's blood. Examples of such parameters include concentrations of carbon dioxide, oxygen, potassium, calcium, and sodium, as well as hematocrit and pH. These signals are supplied to a analyzer, which converts the signals into a form readable by a caregiver.
With blood chemistry monitoring systems as described above, adequate purging of blood from the sensor assembly is a special concern. The accuracy of the blood chemistry measurement can be adversely affected by blood cells and other blood components (e.g., protein) that build up in the various spaces and crevices of the sensor assembly. That is, the accumulation of extraneous blood cells around the sensor electrodes can result in erroneous calibration and inaccurate measurement of blood chemistry. To avoid this problem, successive measurements generally cannot be taken unless the sensor assembly has been thoroughly purged of blood cells between measurements. Thus, a specified purge volume of infusion fluid must be passed through the sensor assembly after each measurement.
For example, such a blood chemistry monitoring system will draw 0.5 to 1.5 ml of blood as a sample. This is considerably more than the minimum amount required to reach the sensors in the sensor assembly, but it ensures that the sensors are exposed to essentially undiluted blood. A method and apparatus for precisely and repeatably drawing a sufficient blood sample to ensure that it reaches all of the sensor assembly's individual sensors and that sufficient additional blood is drawn to minimize the dilution effects of an adjacent infusion fluid is disclosed in co-pending application Ser. No. 08/688,153, filed Jul. 29, 1996, and assigned to the same assignee as the present application. However, such a blood sample then requires about 5 to 10 ml of infusion fluid (i.e., 6 to 10 times the blood drawn for the sample) at rate of 900 ml/hour to purge the sensor assembly. This volume is well within the fluid intake range of most patients, but for volume-restricted adults and pediatric and neonatal patients, this volume is excessive. Furthermore, if a patient requires more frequent monitoring (e.g., every 15 minutes), purging would require up to 1,200 ml of infusion fluid per day.
In an effort to reduce the blood fouling problem described above and the amount of infusion fluid required for purging, it is known to provide a smooth flow path in the sensor assembly. Some assemblies, for example, provide flush-mounted sensor electrodes, in which the electrodes are located in a sensor cavity and a water soluble reference material or gel is placed above the electrode and a polymer-based, selectively-permeable material is placed on top of the gel to form a smooth flow pathway for samples. The selectively-permeable material presents a smooth outer surface to the fluid flow, flush with the electrode housing, and it allows only selected ions to reach the reference gel and thereby produce a reading from the electrode. Such flush-mounted electrode designs, however, can be difficult to manufacture. To provide accurate readings, a precise separation must be provided between the aqueous-based internal reference gel and the polymerbased selectively permeable material, and the electrode must be kept free of any contact with the fluid being measured. Such separation is difficult to maintain due to wicking of the reference gel, which causes it to spread to areas of the sensor cavity where the selectively-permeable layer will be deposited. This can allow the fluid being measured to reach the electrode, shorting the electrode out.
Another proposed solution to the problem is described in the aforementioned U.S. Pat. No. 5,165,406, in which the sensor assembly has an internal conduit with a turbulence structure in the form of a helical groove disposed upstream of the sensor electrodes and in the fluid flow path. This is intended to create turbulence in the flow of fluid through the sensor assembly, which in turn is intended to promote dislodging of any blood cells collected around the sensor electrodes, without appreciably increasing turbulence between the blood and infusion fluid upstream of the sensor assembly when the sample is being taken. However, due to its cost of manufacture and other concerns, this proposed solution has never been implemented in any commercial blood chemistry monitoring system.
From the foregoing discussion, it should be apparent that there is need for more efficient, effective and clinically safe purging of blood samples in infusion fluid delivery systems that avoids collection of blood cells and reduces purge volumes, thereby also reducing the time intervals between successive measurements. The present invention satisfies this need.